Changes in Soil Lignin by the Conversion of Forest to Silvopastoral Systems in Hill-Lands of Appalachia.

Establishment of silvopastoral systems in Appalachia has potential to increase farm income. Required fertilization and liming for the silvopastoral systems modifies soil chemical properties, including formation and stabilization of soil organic matter though the impact is unknown. Lignin is more recalcitrant than other plant polymers and assessment of soil lignin composition in a transition from forest to silvopastoral to pasture systems may be useful as a predictor for long-term soil organic matter stabilization and C sequestration. Soil samples were collected from 0-10 cm depth in the autumn of 2008 from 2 forest sites [dominated by white oak (Quercus alba) or yellow poplar (Liriodendron tulipifera)], 2 matching silvopasture sites (ages being 7 yr for oak and 1 yr for poplar), and 1 open pasture site (~ 40 yr in age) from an Appalachian hill-land farm. Open and silvopasture areas were seeded to orchardgrass (Dactylis glomerata L.), perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) and managed as rotationally grazed systems. Soils were air-dried and lignin-derived oxidation products by copper oxide were analyzed by HPLC. Soil lignin content was higher in the silvopastoral systems than the forest and pasture systems (600±30, 442±25, and 378±17 mg C-lignin/kg soil, respectively) as determined by the sum of the vanillyl (V), syringyl (S), and cinnamyl (C) groups. The acid: aldehyde ratios of V and S groups, indicators of lignin degradation, were highest for pasture soil, followed by soils from forest and silvopastoral systems (0.66, 0.58, and 0.41, respectively). Contribution of woody and non-woody material to the lignin was similar for both silvopastoral and forest soils, as indicated by the low C:V ratios (0.14 and 0.13, respectively), whereas lignin in pasture soil was derived mostly from non-woody material (C:V ratio = 0.36). Results indicated silvopasture soils had highest yet youngest lignin content, derived from both woody and non-woody tissues. This indicated that silvopasture tree residues decomposed faster than in forest systems. Conversely, open pasture had more decomposed and humified lignin.